<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .triangle {
        position: relative;
        width: 100px;
        height: 100px;
        /* background-color: pink; */
      }

      .triangle::before {
        content: "";
        position: absolute;
        top: 0;
        left: 0;
        border-top: 100px solid red;
        border-right: 100px solid transparent;
      }
    </style>
  </head>
  <body>
    <div class="triangle"></div>
  </body>
</html>
